Prior art wireless systems such as 802.11 provide communications where a central node (an “access point”) is coupled to other stations using an “infrastructure mode”. This type of infrastructure system relies on the access point being powered continuously, and the power savings comes about by using, for example, access point beacon frames, where the access point periodically transmits beacon frames at expected times, such that the stations wake up at corresponding intervals of time and listen to beacon TDIM frames which indicate whether packets are available at that time of coming out of a sleep mode. The power consumption is thereby reduced by a factor equal to the small percentage of time the stations are awake compared to the beacon interval. This type of infrastructure works well where the access point is able to be powered by an external power source, and requires that the stations be clustered within receiving range of the access point. FIG. 1A shows an example positioning of stations S1 through S8 and access point AP1. Typical attenuation of wireless signals is as follows (2× indicates a doubling of separation distance between stations):
Line of Sight(LOS) loss6 dB/2x distanceIndoor LOS loss with multi-path12 dB/2x distanceloss per concrete floor traversal15 dB/floor + indoorLOS loss
Accordingly, if the network of FIG. 1A were in open space without barriers and examined in plan view (positioned horizontally in open space), the loss from AP1 to S1 might be 61 dB, AP1 to S2 67 dB, and AP1 to S4 71 dB, whereas for the same dimensions, with FIG. 1A examined as an elevation view with the lines representing concrete floors and including indoor multipath loss, the loss from AP1 to S1 would be 106 dB, from AP1 to S2 would be 157 dB, and from AP1 to S3 would be in excess of 200 dB. If the link budget is on the order of 140 dB, then most of the stations are out of range of the access point.
Another type of network is known as a mesh network, where the stations each communicate with each other as peers. For FIG. 1A, this type of network topology has S1 communicate with AP1 and S2, S2 to S1, S4, S5, and S3, etc. The mesh network has a different set of considerations and issues. One issue is that the network needs to find a route from each station to a gateway such as AP1, and another issue is that there is no centralized timing or transmission as infrastructure mode provides.
Both types of networks, infrastructure and mesh, require access to the internet at large, which is provided by a function known as a gateway router. This functionality is typically offered through an access point AP1 of FIG. 1A, or it may alternatively be offered through a mesh station having connectivity to a gateway router.
Battery powered systems utilize a finite amount of Joule stored energy, such that the Joule product of power*time may be conserved only by either reducing power or reducing the amount of time that power is drawn. When viewed from the perspective of a fixed Joule source and a desired battery lifetime for a given battery size, the problem reduces to one of duty cycle and latency. In the following illustrations of prior art, a fixed source of two AAA batteries is used, which provides 1000 mAh @ 3V. Accordingly, for 10 year operation form this storage source, the average current draw is maximum 10 uA at 3V, which will now be used to compare the performance of various prior art systems.
In the prior art, battery powered network nodes typically use Bluetooth (at a typical 6 mA listen current at 3V) which offers lower power consumption than 802.11 WLAN (at a typical 60 mA listen current at 3V). For a 10 year battery life using two AAA batteries, and the best case of synchronous wakeup intervals across all stations, an average current of 10 uA could be accomplished in Bluetooth (from 5 mA continuous current) by power-cycling the Bluetooth device on for periodic and synchronized listen intervals across all stations, with a duty cycle of 1/500 and a 1 ms on (listen) time. The corresponding approach could be used in WiFi with a duty cycle of 1/6000 and a 1 ms on (listen) time, both would satisfy the 10 uA average current requirement specified. The 10 year 1 AH 3V constraint using two AA cells is used as a uniform baseline example for understanding the invention and its benefits, other power source capacities can be computed using those metrics in the same manner.
For a mesh of 4 stations, where one station has to check with each of the four other stations, each check requiring a 1 ms event wakeup event, the time for each hop is 1 ms*500*4=2 s for Bluetooth and 1 ms*6000*4=24 s for WiFi. Accordingly, just 10 hops through a mesh network has a latency of 20 s (˜4 min) for Bluetooth and 4 minutes for WiFi, which are unacceptable for most purposes.
With four edge nodes (reducing throughput by 4), and where WiFi data throughput is 10 Mbps peak (at 1/6000 duty cycle) and Bluetooth throughput is 250 Kbps (at 1/500 duty cycle), the above examples provide per-day download data of 18 MB for WiFi and 5.4 MB for Bluetooth, which would restrict these uses to very low data transfer applications.
Another problem of the prior art is that for dynamically changing networks, a significant amount of power is consumed with RF advertising, in the case of Bluetooth (Bluetooth Low Energy, known as BLE), where the slave device has an advertising interval and the master device has a scan window and scan interval. As the below table shows, short Bluetooth connection times require the advertising window and scan interval both be short, which greatly increases battery drain. For Bluetooth Long Range (BLR), the problem is exacerbated over BLE by the increased BLR transmit frames and requirement for low duty cycle.
AdvertisingScanScanAvg99%intervalWindowIntervalconnectionconnection(ms)(ms)(ms)time (s)time (s)303030.020.04603030.030.071003030.060.113011.2512802.7112.86011.2512805.5423.0410011.2512808.9235.84128030300.671.37128011.251280162.5323.84
It is desired to provide an apparatus and method for ultra-low power mesh networks which provides bidirectional connectivity from a plurality of peer stations to a gateway router at a higher data rate than the prior art provides.